A rhombus's diagonals bisect each other at their intersection point.
also they intersect at right angles.
halves of both the diagonals and a side form a right angled triangle.
half of the given diagonal is 12/2= 6 cm.
the side's length is 10 cm. (6,8,10) is a Pythagorean triplet, so the half of the other diagonal's length is 8 cm.
the other diagonal's length is 8x2= 16 cm.